Memory disk sheet stock and method

ABSTRACT

There is disclosed an improved memory disk stock comprised of an alloy composition which includes: about 1-11 wt. % magnesium, preferably between about 2-7 wt. % Mg; up to about 0.3 wt. % zirconium preferably about 0.02-0.25 wt. % Zr; up to about 0.2 wt. % iron, up to about 0.2 wt. % silicon, and about 0.02-0.2 wt. % of a dispersoid-forming element selected from scandium, erbium, thulium, lutetium, ytterbium, hafnium and yttrium, the balance aluminum and incidental elements and impurities. With preferably about 0.05-0.15 wt. % of scandium added to such compositions, these disk stocks exhibit significant room temperature yield strength increases over the same compositions without any scandium added.

This application is a continuation-in-part of U.S. application Ser. No.08/299,836, filed Sep. 1, 1994 now U.S. Pat. No. 5,554,428, thedisclosure of which is fully incorporated by reference herein.

BACKGROUND OF THE INVENTION:

This invention relates to the field of magnetic disk substrates. Moreparticulary, the invention relates to an improved aluminum-based alloymemo disk sheet stock, magnetic recording substrate products madetherefrom and a related method for making such memo disk products.

The hard magnetic disks used as memo media for data storage in computersrequire an extremely high quality material. Aluminum, specificallyaluminum alloys, are a metal of choice, due to the relatively highstrength, light weight, low cost and good surface finishingchacteristics of such alloys. Aluminum-magnesium alloys have been usedfor some time as substrates for the magnetizable layers of many memorydisks. U.S. Pat. Nos. 4,699,672, 4,722,872 and 4,751,958 arerepresentative of the magnesium-containing, aluminum-based alloys usedfor such memory storage devices. Various ways to manufacture such sheetproducts are disclosed in these references. Some of the more commonaluminum-based alloys currently used for this purpose include 5082, 5086and 5182 aluminum (Aluminum Association designations). These alloys allhave magnesium ranges within around 3.5-5.5 wt. %.

Advances in memo storage technology may necessitate further reductionsin thickness and/or size of the magnetizable disk, or at least themagnetizable layers deposited on a disk substrate. At the same time, newtechnologies demand an increase in the density of information storableper disk.

SUMMARY OF THE INVENTION

It is a principal objective of this invention to provide an aluminummemory disk sheet stock with an improved combination of properties,especially platability, flatness retention and higher room or lowtemperature yield strengths, in excess of 30 ksi and, more preferably,between about 32-35 ksi, although typical room temperature yieldstrength values of about 40 ksi or better are also possible with thepresent invention. It is another main objective of this invention toconsistently impart such improved properties to memory disk stock so asto enable the designers of future computer and other magnetic mediastorage devices to accommodate smaller, thinner disks which can storethe same or even greater amounts of information thereon.

It is yet another main objective of the present invention to provide astronger magnesium-containing, aluminum-based alloy disk substrate whichmay be shrunk in size and/or thickness because of the strength gainsotherwise achieved. Yet another main objective is to providealuminum-based memory disk stock with an ability to maintain itsflatness better and longer, especially through higher thermal treatmenttemperatures. Still another main objective is to strengthen existingcompositions used in the manufacture of aluminum-magnesium memory disksso as to make such products less vulnerable to damage during subsequentprocessing and/or end use.

These and other objectives are met or exceeded by the present invention,one embodiment of which pertains to an improved memory disk stockcomprised of an alloy composition which includes: about 1-11 wt. %magnesium, preferably between about 2-7 wt. % Mg; up to about 0.3 wt. %zirconium, preferably between about 0.02-0.25 wt. % Zr; up to about 0.2wt. % iron, preferably less than about 0.03 wt. % Fe; up to about 0.2wt. % silicon, preferably less than about 0.05 wt. % Si; and about0.02-0.2 wt. % of an element which forms a recrystallization-inhibitingcompound, said element selected from the group consisting of scandium,erbium, thulium, lutetium, ytterbium, hafnium and yttrium, the balancealuminum and incidental elements and impurities. Preferably, about0.05-0.15 wt. % of scandium exists in the alloy compositions of thisinvention. Magnetic media recording devices formed from these disk sheetstock have average room or low temperature yield strengths of about 30ksi or more, typically between about 32-35 ksi. There is also claimed animproved method for making memory disk devices by adding about 0.02-0.2wt. % of scandium, erbium, thulium, lutetium, ytterbium, hafnium and/oryttrium to new or existing magnesium-containing, and preferablyzirconium-containing, aluminum-based alloys.

BRIEF DESCRIPTION OF THE DRAWING

Further features, objectives and advantages of this invention willbecome clearer in the following detailed description of preferredembodiments made with reference to the accompanying FIG. 1 in whichtypical room temperature yield strengths are plotted versus scandiumweight percentages for the various invention alloys listed in the Table1 data below.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For the description of alloy compositions that follows, all referencesto percentages are by weight percent (wt. %) unless otherwise indicated.When referring to any numerical range of values, such ranges areunderstood to include each and every number and/or fraction between thestated range minimum and maximum. A range of about 0.02-0.2 wt. %scandium, for example, would expressly include all intermediate valuesof about 0.021, 0.022, 0.023 and 0.03%, all the way up to and including0.197, 0.198 and 0.199 wt. % Sc. The same applies to each otherelemental range set forth below.

As used herein, the term "substantially-free" means no significantamount of that component has been purposefully added to the alloy, itbeing understood that trace amounts of incidental elements and/orimpurities may still find their way into a desired end product. Forexample, a substantially lead-free, disk sheet stock may contain lessthan about 0.1% Pb, or less than about 0.03% Pb on a more preferredbasis, due to contamination from incidental additives or through contactwith certain processing and/or holding equipment. All embodiments of thepresent invention are substantially Pb-free. The invention alloy is alsosubstantially free of bismuth, nickel, vanadium, beryllium, cadmium andthallium on a most preferred basis.

While this detailed description focuses on adding scandium to new orexisting aluminum-based, memory disk stock, it is to be understood thatthe invention should not be limited to this metal addition alone. Asused herein, scandium is a representative "rare earth metal" even thoughit doesn't appear with the other lanthanide metals listed in Group IIIBof the Periodic Table. Nor is yttrium automatically grouped with thissame series of elements even though it often performs the same functionas scandium, or other "true" rare earths in an alloy composition. It isbelieved that minor amounts of still other rare earths, like erbium,thulium, lutetium, ytterbium, or another rare earth "act-alike", likehafnium, may be substituted for, or possibly even combined with scandium(or with each other) in varying quantities to achieve the substantialstrength level improvements observed herein. For at least one particularaluminum-magnesium alloy, a two- to three-fold increase in strength wasobserved for the same composition made with scandium versus that samecomposition without any scandium added thereto.

In the same way, this invention focuses on making improved substratesfor magnetic media recordation, i.e., memory disks and the like. Inpractice, that means forming the entire body of a disk whose outersurface may have a thin metal, metal oxide or even polymer componentapplied thereon. It is to be understood, however, that the metalcompositions of this invention may also be used in conjunction withexisting or currently developmental disk manufacturing techniques,including but not limited to making disks from metal clad productsand/or from gradient components as in a spray formed product whosecomposition changes either through the thickness of the product, fromits inner to its outer circumference, or both.

"Platability", as used herein, means the ability of an aluminum disksubstrate to allow electro- or electroless plating solutions and/orpretreatments to be deposited thereon so as to form a smooth depositlayer on an outer surface of this substrate, said deposit layer beingsubstantially free of pits and other surface defects.

When referring to preferred aging, annealing and/or tempering treatmentsfor this invention, including but not limited to: H19, H1X, H2X, H3X, orT6, T8, T851 and T9 tempers, it is understood that current practicesinclude: hot working; cold working; and precipitation hardening, eitherat room or ambient temperatures (i.e. naturally) or artificially (usingan external heat source) for effecting a partial anneal thereof.Particulars about such annealing and/or tempering processes are all verywell known and can be found, for instance, in Aluminum Associationguidelines like the 1993 publication called Aluminum Standards and Datawhich is also incorporated by reference herein.

While not being limited to any particular theory, it is believed thatthis invention manages to impart significantly higher strengths tomemory disk stock through the addition of certain rare earths or rareearth "act alikes", preferably scandium, by causing rare earth-richprecipitates to form said precipitates having the ability to store andresist plastic deformation and, due to the relatively small size andfine distribution of these particles, recovery and recrystallization ofthe resulting alloy are inhibited. However, at scandium concentrationsin excess of about 0.15 wt. %, larger particles of Al₃ Sc start toprecipitate during subsequent heat treatment. And while suchprecipitation does not necessarily detrimentally effect room temperatureyield strength values for the above-described, preferred alloycomposition, their dropping out can have a detrimental effect on platingresponse.

The same alloy is also more temperature resistant than the same alloywhich is devoid of scandium or scandium like additives. By "temperatureresistant" or "temperature resistance", it is meant that a large portionof the strength and structure imparted by working this alloy is retainedin the memory disk substrate, even after exposure to one or more highertemperatures, typically above about 450° F., such as during subsequentflattening operations or the like. Specimens of this disk sheet stockhave been examined by mechanical testing and subjected tomicrostructural analysis to observe this feature.

When referring to the main alloying components of this invention, it isunderstood that a remainder of substantially aluminum may include someincidental, intentionally added elements which may affect collateralproperties of the invention, or unintentionally added impurities,neither of which should change the essential characteristics of thisalloy. With respect to these main alloying elements of the morepreferred embodiments of this invention, it is believed that magnesiumcontributes to strain hardening and strength. Scandium contributes tothe artificial aging response of memory disk products made from thepresent invention. And zirconium additions are believed to improve theresistance of scandium precipitates to undergo rapid growth in preferredembodiments of this alloy. Together, the scandium and zirconium serveyet another purpose. When added to aluminum-magnesium alloys of the typedescribed herein, scandium will precipitate to form a dispersion offine, intermetallic particles (referred to as "dispersoids"), typicallyof an Al₃ X stoichiometry, with X being Sc, Zr or both Sc and Zr in thecase of scandium and zirconium additions. Al₃ Sc dispersoids impart somestrength benefit as a precipitation-hardening compound, but moreimportantly, such dispersoids are very efficient at retarding theprocesses of recovery and recystallization. These processes are impededby a phenomenon that is sometimes called the "Zener Drag" effect. [Seegenerally, C. S. Smith, TMS-AIME, 175, 15 (1948).] It results becausethe scandium dispersoids are very small in size, but also large innumber. They generally retard recovery and recrystallization by actingas pinning points for migrating grain boundaries and dislocations whichmust bypass them in order for the metal to soften. Recrystallization andrecovery are the principal metallurgical processes by which such strainhardenable alloys soften. In order to "soften" an alloy having a largepopulation of Al₃ Sc particles, one would have to heat the material tomuch higher temperature than would be required for an alloy not havingsuch particles present. Put another way, when identicallystrain-hardened and annealed, a sheet product (or in this case memorydisk stock) that contains Al₃ Sc dispersoids will have higher strengthlevels than a comparable alloy without any scandium present. For memorydisks, a further benefit is this sheet product's ability to resistsoftening through the high temperature thermal exposures usually neededto flatten sheet product of this sort and then sputter magnetic media onthe substrate. In so doing, the invention alloy will retain some of thestrengths it acquired through the rolling operations performed thereon.Other prior art alloys, without Sc added thereto, tend to soften throughthese same flattening treatments thus yielding a lower strength, finalproduct. An added benefit of zirconium is its ability to limit thegrowth of Al₃ (Sc, Zr) particles to assure that such dispersoids remainsmall, closely spaced and capable of producing the Zener Drag effect.

It is preferred to minimize the effects of certain impurities in thesealloy compositions. Impurities, like iron and silicon, can have adeleterious effect on the quality of memory disks produced from thisfeedstock, especially if the memory disk substrate is to be plated. Theyare less critical where no subsequent plating is anticipated. In anyevent, it is preferred that Fe and Si levels be kept especially low,below 0.5 wt. % each, preferably below about 0.1 wt. % each, and mostpreferably at or below about 0.03 wt. % for iron and about 0.05 wt. %for silicon.

On a less preferred basis, the alloy disk stock of this invention mayhave readily observable strength level improvements imparted to itthrough the additions of scandium, erbium, thulium, lutetium, ytterbium,hafnium and/or yttrium when these same alloys contain such staplealloying components as zinc, up to about 5 wt. %, typically for naturalor artificial aging response enhancement, and up to about 1 or 2 wt. %copper for improving that alloy's precipitation and solid solutionstrengthening performance. For these reasons, it is consideredappropriate to more generally identify the present invention by itsbroadest, most generic form, namely, adding one or moredispersoid-forming, rare earth metal, preferably scandium, or rare earthmetal "act alikes" to a new or existing aluminum-based memory disk stockproduct composition.

In the typical processing of memory disks from alloy stock, there areincluded the usual steps of: (a) providing an aluminum-based alloy; (b)making thin sheet product from this alloy, by casting, rolling, or othermetallurgical routes including spray forming; (c) cutting or stamping(or sometimes called "blanking") disk substrates from the thin sheetproduct; and finally (d) converting said disk substrates into computermemory disks, usually by an elaborate number of substeps includingflattening, polishing and/or thermally heat treating the final product.

The typical sizes of aluminum-based memory products manufactured todayrange from about 1 to 5 inches in overall diameter and from about0.020-0.100 inch in overall thickness. While it cannot be predicted towhat extent these sizes and/or thicknesses may be reduced by future diskstorage designers, it is believed that the strength increase impartedthrough the addition of scandium according to this invention will givesuch designers enhanced flexibility in how they conceive and producefuture magnetic data storage media.

When platability of an intermediate product does not meet specification,there is a certain amount of grinding which can be performed on a diskto reduce its high points and bring its overall outer surface into amore uniform, useable state. Alternately, it may be possible tothermally flatten a product by heat treating to reduce or preferentiallyeliminate any residual stresses in the substrate which may bedetrimental to its flatness. Such a treatment may be performed in thatalloy product's annealing temperature range, generally from about500°-750° F. for most 5XXX or 7XXX Series aluminum alloys, sometimeswith a number of noncomplying disks stacked together under compression.

The following example is provided to further illustrate the objectivesand advantages of this invention. It is not intended to limit the scopeof the present invention in any manner, though.

EXAMPLES

Several ingots were cast to compare the effect of scandium concentrationon various properties of the invention alloy, including typical roomtemperature yield strength. Each of the compositions listed belowcontained roughly 4 wt. % magnesium, 0.1 wt. % zirconium, 0.03 or lesswt. % iron and 0.05 or less wt. % silicon. The scandium levels were thenvaried as per the Table 1 data that follows. Each such composition wascast and rolled to a final gauge of about 0.035 inch. Portions of thisproduct were then divided and subjected to a thermal treatmentconsisting of exposure for 7 hours to temperatures of about 625° F., afairly standard heat treatment for products of this sort. The roomtemperature yield strengths of these compositions were then measured asfollows:

                  TABLE 1                                                         ______________________________________                                        Sample No. Wt. % Scandium                                                                             Yield Strength (ksi)                                  ______________________________________                                        1          0.05         30.0                                                  2          0.07         32.0                                                  3          0.09         32.5                                                  4          0.10         32.5                                                  5          0.13         33.0                                                  6          0.15         35.0                                                  7          0.18         35.0                                                  ______________________________________                                    

The strength values of Table 1 are plotted in accompanying FIG. 1.

Having described the presently preferred embodiments, it is to beunderstood that the invention may be otherwise embodied within the scopeof the appended claims.

What is claimed is:
 1. A memory disk sheet stock having improvedstrength levels, said memory disk stock comprising an alloy compositionwhich consists essentially of about 1-11 wt.% magnesium, up to about 5wt. % zinc, up to about 1 wt. % copper, up to about 0.3 wt. % zirconium,up to about 0.2 wt. % iron, up to about 0.2 wt. % silicon, and about0.02-0.2 wt. % of a dispersoid-forming element selected from the groupconsisting of: scandium, erbium, thulium, lutetium, ytterbium, hafniumand yttrium, the balance aluminum and impurities.
 2. The memory disksheet stock of claim 1 which has an average room temperature yieldstrength greater than about 30 ksi.
 3. The memory disk sheet stock ofclaim 2 which has an average room temperature yield strength betweenabout 32-35 ksi.
 4. The memory disk sheet stock of claim 1 wherein thedispersoid-forming element is scandium.
 5. The memory disk sheet stockof claim 4 which contains about 0.05-0.15 wt. % scandimn.
 6. The memorydisk sheet stock of claim 1 which contains about 2-7 wt. % magnesium andless than about 0.1 wt. % zirconium.
 7. The memory disk sheet stock ofclaim 1 which contains less than about 0.03 wt. % iron and less thanabout 0.05 wt. % silicon.
 8. A magnetic recording disk substratecomprising an alloy composition which consists essentially of about 1-11wt. % magnesium, up to about 5 wt. % zinc, up to about 1 wt. % copper,about 0.02-0.3 wt. % zirconium, up to about 0.2 wt. % iron, up to about0.2 wt. % silicon, and about 0.02-0.2 wt. % scandium, the balancealuminum and impurities.
 9. The disk substrate of claim 8 which has anaverage room temperature yield strength greater than about 30 ksi. 10.The disk substrate of claim 9 which has an average room temperatureyield strength between about 32-35 ksi.
 11. The disk substrate of claim8 which contains about 0.05-0.15 wt. % scandium.
 12. The disk substrateof claim 8 which contains about 2-7 wt. % magnesium and less than about0.1 wt. % zirconium.
 13. The disk substrate of claim 8 which containsless than about 0.03 wt. % iron and less than about 0.05 wt. % silicon.14. In a method for making computer memory disks from amagnesium-containing aluminum-based alloy composition, said methodcomprising the steps of: (a) providing an aluminum-based alloy having atleast about 2 wt. % magnesium; (b) making thin sheet product from saidaluminum-based alloy; (c) cutting disk substrates from said thin sheetproduct; and (d) converting said cut disk substrates into computermemory disks, the improvement which comprises adding to saidaluminum-based alloy between about 0.02-0.2 wt. % of an element selectedfrom the group consisting of: scandium, erbium, thulium, lutetium,ytterbium, hafnium and yttrium.
 15. The improvement of claim 14 whereinsaid included element is scandium.
 16. The improvement of claim 14wherein said aluminum-based alloy contains about 0.05-0.15 wt. % ofscandium.
 17. The improvement of claim 14 wherein said memory disks havean average room temperature yield strength of about 30 ksi or more. 18.The improvement of claim 17 wherein said memory disks have an averageroom temperature yield strength between about 32-35 ksi.